The present invention relates to an electrophoretic display having improved stability and to a method of making the display using a micro-molding technique.
An electrophoretic display essentially comprises a suspension of charged colored particles (typically white) in an optically dense liquid of another color. The suspension is maintained between two electrodes that define a sealed cell. It is xe2x80x9coptically densexe2x80x9d in the sense that the suspension medium is sufficiently colored that one cannot see from one side of the cell to the other. When a potential difference exists between the electrodes, the particles are driven away from one of the electrodes towards the other electrode. When driven to the side of the cell nearest the viewer, the color of the particles will dominate the color of the display. Conversely, when the particles are driven to the far side, away from the viewer, the color of the liquid dominates the color of the display.
For example, an electrophoretic display device is illustrated in FIG. 1. This device comprises a sealed cell 10 formed with two closely-spaced plates 12, 14. At least one of the plates 12 defining the front of the device is glass or otherwise transparent. Electrodes are disposed on the plates, e.g., a first transparent electrode 13 is on the front glass plate 12 and a second electrode 15 is on the rear plate or substrate 14. At least one of the electrodes should be transparent, although they may both be transparent. The cell 10 holds an optically dense liquid 20 in which is suspended a plurality of charged particles 18, etc., which are shown as negatively charged. The particles may be a light color with the suspension medium 20 being a dark color, e.g., it typically is comprised of diarylide yellow pigment dispersed in a solvent such as tetrachloroethylene and xylene having a colored dye dissolved therein.
In operation, when a negative charge is applied to the rear electrode 15, the particles are driven toward the front electrode 13, such that the viewer will see the color of the particles through the front glass panel 12. However, with an opposite applied potential, the particles travel to the back of the cell and are obscured from view, such that the color of the liquid 20 determines the color of the cell. Color combinations of pigmented particles and the liquid may be used to achieve a desired color display, and a mixture of colored particles having different charges can be used with applied AC voltage to achieve color variations. Displays applying these principles are non-emissive, bistable (and hence power efficient), and can be fabricated at low cost, over large areas, and on flexible substrates. Such electrophoretic display devices and their properties are further described in A. Dalisa, xe2x80x9cElectrophoretic Display Technology,xe2x80x9d IEEE Transactions on Electron Devices, Vol. ED-24, No. 7 (July 1977), at p. 827; I. Ota et al., xe2x80x9cElectrophoretic Image Display (EPID) Panel,xe2x80x9d Proceedings of the IEEE, Vol. 61, No. 7 (July 1973), at p. 832; Fitzhenry-Ritz, xe2x80x9cOptical Properties of Electrophoretic Image Displays,xe2x80x9d IEEE Transactions on Electron Devices, Vol. Ed-28, No. 6 (June 1981), at p. 726, which are incorporated herein by reference.
A drawback with these devices that has inhibited their commercialization is that their lifetimes are limited. Typically, the visible appearance of the display degrades after a few thousand hours. Pigment clustering and agglomeration are common modes of degradation. By xe2x80x9cagglomeration,xe2x80x9d it is meant that the particles tend to become nonuniformly distributed in the plane of the display on length scales visible to the naked eye. This effect degrades the appearance and resolution of the display. LCDs and LEDs have been commercialized in preference to electrophoretic displays (EPDs). See, e.g., B. Comiskey et al., xe2x80x9cAn Electrophoretic Ink for All-Printed Reflective Electronic Displays,xe2x80x9d NATURE Vol. 394 (Jul. 16, 1998), at p. 253, reporting that while xe2x80x9cmicroparticle-based displays have long intrigued researchers, . . . such displays have to date suffered from short lifetimes and difficulty in manufacture.xe2x80x9d
The difficulties with agglomeration are even more problematic in displays specially designed for achieving good image contrast. U.S. Pat. No. 5,872,552 issued Feb. 16, 1999, to Gorden et al., xe2x80x9cElectrophoretic Display,xe2x80x9d assigned to IBM Corp. (incorporated herein), discloses a structure designed to achieve good image contrast by using an electrode at the side of the cell, a pedestal-shaped counter electrode within the cell, a front transmissive window, and optionally, a rear reflective surface. A difficulty with the Gordon configuration is that the counter electrode, being an electrically active component connected to the power supply, attracts the particles to a particular area of the cell and thereby increases the likelihood of agglomeration. See also U.S. Pat. No. 4,655,897 to DiSanto and U.S. Pat. No. 4,203,106 to Dalisa et al., both of which show rectangular-shaped electrodes that present the likelihood of agglomeration.
A number of techniques recently have been developed for achieving more stable EPDs but each has significant disadvantages. One approach, for example, involves encasing the charged particles and supporting liquid in substantially spherical polymeric xe2x80x9cmicrocapsules.xe2x80x9d See, e.g., PCT patent application Ser. No. PCT/US98/04705, published Sep. 24, 1998, titled xe2x80x9cImproved Microencapsulated Electrophoretic Display,xe2x80x9d and PCT patent application Ser. No. PCT/US97/18643, published May 7, 1998, titled xe2x80x9cNonemissive Displays and Piezoelectric Power Supplies Therefor,xe2x80x9d incorporated herein.
The microcapsules are closed microscopic vessels fabricated using coacervation, interfacial polymerization, or in-situ polymerization. The microcapsules do not entirely prevent agglomeration but they confine it within single capsules, which typically are too small to be seen by eye. Although this technique minimizes the adverse effects of agglomeration, it has disadvantages. For instance, fabrication of the microcapsules involves polymerization schemes carried out in the presence of the colored liquids and charged particles. Precise control must be exercised over the processing conditions such as the temperature, pH, and starting material concentrations, and it is difficult to control the size or uniformity of the microcapsules. These polymerization schemes place limits on the materials that may be used for the device and require additional fabrication steps. The starting materials, any intermediates, and of course, the end products need to be chemically compatible with the rest of the device, e.g., the materials used for the pigment particles and suspension medium, as the fabrication is performed in-situ.
Another approach for addressing agglomeration involves placing a charge on the pigment particles. For example, U.S. Pat. No. 5,403,518, xe2x80x9cFormulations for Improved Electrophoretic Display Suspensions and Related Methods,xe2x80x9d issued Apr. 4, 1995 to Schubert and assigned to Copytele Inc., uses a charge control agent adsorbed on the pigment particles for preventing agglomeration. U.S. Pat. No. 4,680,103, issued Jul. 14, 1987 to Beilen, xe2x80x9cPositive Particles in Electrophoretic Display Device Composition,xe2x80x9d describes attaching an organosilane to each of the particles where the organosilane includes a positively charged ionic functional moiety covalently bonded therein.
Other approaches involve using pulsed and DC voltages to periodically redistribute the particles or use of xe2x80x9celectrostatic compartmentsxe2x80x9d to restrict particle movement to a defined region. These approaches, if effective, require significant increases in the complexity of the electrodes and drive circuitry. S. Beilin et al., xe2x80x9c2000 Character Electrophoretic Display,xe2x80x9d SOCIETY OF INFORMATION DISPLAY 86 Digest (1986), at p. 136. However, these approaches have never been demonstrated to be sufficiently effective for commercial devices with long lifetime.
As may be appreciated, techniques for improving the performance and lifetimes of EPDs are desired. The feasibility of commercializing EPDs depends on the development of effective approaches for eliminating agglomeration.
The applicants have discovered a configuration for an electrophoretic display device that is effective in substantially preventing agglomeration of the pigment particles. With this configuration, a suspension medium is contained between first and second electrodes to define a cell; a plurality of pigment particles are dispersed in the suspension medium; and a plurality of mechanical members project in the suspension medium. The members advantageously are sized sufficiently small that a plurality of members can exist within a minimally visible region of the cell, and they are configured (e.g., depending on size and materials), to have no adverse impact on the appearance of the display. The mechanical members may be fabricated separately from the remaining parts of the display, allowing for flexibility in the fabrication materials and processing conditions.